It's unclear whether Trump was quoting the Chinese president or had misunderstood what he was told when he said Korea had been part of China

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This April 7, 2017, file photo showsPresident Donald Trump (left) and Chinese President Xi Jinping in West Palm Beach, Florida.

U.S. President Donald Trump's apparently offhand comment after meeting with Chinese leader Xi Jinping — that "Korea actually used to be a part of China" — has enraged many South Koreans.

The historically inaccurate sentence from a Wall Street Journal interview bumps up against a raft of historical and political sensitivities in a country where many have long feared Chinese designs on the Korean Peninsula. It also feeds neatly into longstanding worries about Seoul's shrinking role in dealing with its nuclear-armed rival, North Korea.

Ahn Hong-seok, a 22-year-old college student, said that if Trump "is a person capable of becoming a president, I think he should not distort the precious history of another country."

Many here assume that Xi fed that ahistorical nugget to Trump, who also admitted that after 10 minutes listening to Xi, he realized that Beijing's influence over North Korea was much less than he had thought.

Here's why Trump's comments strike a nerve in South Korea:

WRONG, BUT WHOSE MISTAKE?It's unclear whether Trump was quoting Xi or had misunderstood what he was told when he said Korea had been part of China.

It never was, historians outside of China say, although some ancient and medieval kingdoms that occupied the Korean Peninsula offered tributes to Chinese kingdoms to secure protection. And for a period during the 13th century, both China and Korea were under the rule of the Mongolian empire.

"Throughout the thousands of years of relations, Korea has never been part of China, and this is a historical fact that is recognized internationally and something no one can deny," Cho June-hyuck, a South Korean Foreign Ministry spokesman, said Thursday.

HISTORICAL FEUDTrump stumbled into a long history dispute between the Asian neighbors; specifically, their views over the dominion of ancient kingdoms whose territories stretched from the Korean Peninsula to Manchuria.

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South Koreans see these kingdoms as Korean, but China began to claim them as part of its national history in the early 1980s.

At the time, China's state historians were exploring ways to ideologically support Beijing's policies governing ethnic minorities, including the large communities of ethnic Koreans in the northeast, experts say.

In the early 2000s, a Chinese government-backed academic project produced a slew of studies arguing that the kingdom of Goguryeo (37 B.C.-A.D. 668) was a Chinese state. This infuriated South Korea, where nationalists glorify Goguryeo for its militarism and territorial expansion. Seoul launched its own government-backed research project on Goguryeo in 2007.

Some analysts say the argument is more political than historical as Goguryeo existed more than a thousand years before the foundation of modern states in Korea and China.

'KOREA PASSING'Several South Korean newspapers mentioned the Chinese claims over Goguryeo as they lashed out at Trump over the comments, and at Xi for allegedly feeding the U.S. president Chinese-centric views.

Chosun Ilbo, South Korea's largest newspaper, said China was looking to "tame" South Korea and weaken the traditional alliance between Seoul and Washington in an attempt to expand its regional influence.

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Seoul has long worried about losing its voice in international efforts to deal with North Korea's nuclear threat — something local media have termed "Korea Passing." Seoul and Beijing are also bickering over plans to deploy in South Korea an advanced U.S. missile defense system that China sees as a security threat.

In the meantime, Trump has reportedly settled on a "maximum pressure and engagement" strategy on North Korea, which is mainly about enlisting the help of Beijing to put pressure on Pyongyang.

"It's highly possible that China will try to solve the problems surrounding the Korean Peninsula based on a hegemonic stance that likens the Koreas to Chinese vassal states," said the Munhwa Ilbo newspaper on Thursday. "If Trump has agreed with this view, you will never know what kind of a deal the two global powers will make over the fate of the Korean Peninsula."